The exhaust nozzle of a jet engine is typically constructed of a high temperature material to withstand the relatively high temperature of combustion gases outflowing from the engine core. For example, many exhaust nozzles are conventionally constructed of titanium which has a favorable strength-to-weight ratio and favorable mechanical properties at elevated temperatures. However, the trend in jet engine design is an increase in combustion gas temperature in the exhaust outflow. Such increased combustion gas temperatures may approach the operating limits of titanium.
Sandwich structure may be used in high-temperature structural applications because of the high specific strength and relatively low weight of sandwich structure. Conventional sandwich structure may be comprised of a pair of opposing face sheets separated by a core material. Conventional core material in a conventional sandwich structure may provide a relatively high level of stiffness in multiple directions. For example, conventional core material may provide a relatively high level of stiffness in a lengthwise or longitudinal in-plane direction of the sandwich structure and in a widthwise or transverse in-plane direction of the sandwich structure. The core material may also provide extensional stiffness in an out-of-plane direction of the sandwich structure.
Unfortunately, when a sandwich structure is initially subjected to a high temperature environment such as during engine startup of a gas turbine engine when hot combustion gas flows over the exhaust nozzle, one face sheet may be rapidly heated to a higher temperature than the opposing face sheet. The resulting increase in temperature of the heated face sheet may result in an expansion of the face sheet in an in-plane direction. The in-plane expansion of the heated face sheet may result in significant stress in both face sheets due to the shear coupling of the face sheets by the stiff core.
As can be seen, there exists a need in the art for a system and method for providing a sandwich structure that provides a high level of strength and stiffness while simultaneously accommodating in-plane expansion of one of the face sheets of the sandwich structure due to differential heating of the sandwich structure.